1. Field of the Invention
The present invention relates to photoresist polymers and photoresist compositions comprising the same. Photoresist polymers and photoresist compositions of the present invention are suitable for a photolithography process employing a deep ultraviolet light source, preferably vacuum ultraviolet(VUV, 157 nm) or extreme ultraviolet (EUV, 13 nm).
2. Description of the Background Art
A photoresist (PR) polymer for an ArF or VUV photolithography process should have a variety of physical characteristics, such as low absorbency of 193 nm and 157 nm wavelengths, and excellent etching resistance and adhesiveness. In addition, the photoresist should be easily developable in a commercially readily available developing solution, such as 2.38 wt % or 2.6 wt % aqueous tetramethylammonium hydroxide (TMAH) solution.
There has been much research done on resins having a high transparency at a wavelength of 193 nm and dry etching resistance similar to Nobolac resin. However, most of the photoresists are not suitable for VUV due to their poor transmittance at 157 nm wavelength. Photoresists containing fluorine and silicon have good transmittance at these wavelengths. Unfortunately, most photoresists containing fluorine with a polyethylene or polyacrylate polymer backbone have weak etching resistance, low solubility in an aqueous TMAH solution and poor adhesiveness to the silicon wafer. In addition, these photoresists are difficult to mass-produce and are expensive. Furthermore, during a post-exposure bake (PEB) process these photoresist can generate HF which can contaminate a lens or corrode a device. Thus, these photoresists are generally not suitable for commercial use. On the other hand, photoresists containing silicon require a 2-step process such as HF treatment and O2 treatment during the etching process. And because it is difficult to remove HF completely, these types of photoresists are generally not suitable in the production of semiconductor devices.
An object of the present invention is to provide photoresist polymers having a good transmittance at wavelengths of 157 nm or 13 nm, etching resistance, adhesiveness and photosensitivity, and a process for preparing the same.
Another object of the present invention is to provide photoresist compositions comprising the PR polymers described above, and a process for preparing the same.
Still another object of the present invention is to provide a semiconductor element produced by using the photoresist composition.
One aspect of the present invention provides a photoresist polymer of the Formula: 
wherein
Y is H;
R1 is a hydroxy protecting group; and
the ratio of a:b:c is 10-90 mol %:5-45 mol %:5-45 mol %, preferably 50 mol %:5-45 mol %:5-45 mol %. The terminal groups in Formula 4 depends on the polymerization initiator and polymerization terminator.
As used throughout this disclosure, it should be appreciated that the order of each monomeric units represented in a polymer formula of the present invention does not necessarily indicate the actual order of such monomeric units in the actual polymer. The monomeric units represented in polymer formulas are intended to simply indicate the presence of such monomeric unit in the polymer. Moreover, the variables represent the total relative ratio of each units. For example, the total amount xe2x80x9caxe2x80x9d of polymeric units derived from norborylene of Formula 4 above can be inter dispersed throughout the polymer (not necessarily in same concentrations) or all or majority of such polymeric unit can be concentrated in one particular location of the polymer.
Variety of hydroxy protecting groups are known to one of ordinary skill in the art. Exemplary hydroxy protecting groups can be found in, for example, Protective Groups in Organic Synthesis, 3rd edition, T. W. Greene and P. G. M. Wuts, John Wiley and Sons, New York, 1999, which is incorporated herein by reference in its entirety. Preferably, the hydroxy protecting group is an acetal. More preferably, the hydroxy protecting group is of the formula: xe2x80x94CH(CH3)OR4, where each R4 is independently optionally substituted linear or branched (C1-C20) alkyl or aryl, or 5-7 membered cyclic lactam.
Examples of preferred polymers of the present invention include, but are not limited to: 
wherein the ratio of a:b:c is 10-90 mol %:5-45 mol %:5-45 mol %.
Polymers of the present invention can be prepared by a variety of methods. In one particularly preferred method, polymers of the present invention are prepared by (1) synthesizing a copolymer of alicyclic monomer and maleic anhydride (or a derivative of maleic anhydride; (2) esterifying the copolymer; and (3) reducing the esterified copolymer to lower light absorbency in the wavelength of 157 nm.
In one particular embodiment of the present invention, a process for preparing photoresist polymer of the Formula 4 comprises the steps of:
(a) polymerizing norbornylene and a maleic anhydride derivative to produce a polymer of Formula 1;
(b) contacting the polymer of Formula 1 with an alcohol to produce a polymer of Formula 2;
(c) contacting the polymer of Formula 2 with a reducing agent to produce a polymer of Formula 3; and
(d) reacting the polymer of Formula 3 with a hydroxy protecting group precursor to produce at least a partially hydroxy protected polymer of Formula 4. 
wherein,
Y is H; and
the ratio of a:d is 10-90 mol %:10-90 mol %.
The alcohol in step (b) is preferably a lower alcohol of the formula R5OH, where R5 is (C1-C5) alkyl, more preferably, the alcohol is methanol. The degree of esterification is generally determined by the amount of R5OH used. For example, when an excess amount of R5OH is used in step (b), theoretically all the carboxylic acid groups in Formula 1 can be esterified. However, it is possible that 5% or less of carboxylic acid group remains unesterified depending on a variety of factors such as reaction conditions including the purity of R5OH.
In one embodiment, the reducing agent in step (c) is preferably NaBH4.
Alternatively, the polymer of Formula 3 can be prepared from the polymer of Formula 1 by reducing the polymer of Formula 1 directly. In such a process, the reducing agent is preferably LiAlH4.
As used herein, the term xe2x80x9chydroxy protecting group precursorxe2x80x9d refers to a compound which when reacted with a hydroxy functional group results in the protection of the hydroxy functional group. Such hydroxy protecting group precursors are well known to one of ordinary skill in the art and include compounds that are disclosed in the above disclosed Protective Groups in Organic Synthesis, 3rd edition, which was previously incorporated by reference in its entirety. In one particular embodiment, the hydroxy protecting group precursor is a (C1-C20) alkyl or aryl vinyl ether compound, including tert-butylvinylether, ethylvinylether and cyclohexylvinylether.
The polymerization can be conducted in an organic solvent. Preferably, the polymerization solvent is selected from the group consisting of tetrahydrofuran, dimethylformamide, dimethylsulfoxide, dioxane, benzene, toluene and xylene. The polymer can be separated and/or purified by crystallization. Preferably the polymer crystallization solvent is selected from the group consisting of diethylether, petroleum ether, water, lower alcohols (such as methanol, ethanol, and isopropanol), and mixtures thereof.
One particular embodiment for preparing photoresist polymers of the present invention is illustrated in Reaction Scheme 1: 
where R1, R5, Y, a, b, c, and d are those defined above.
Another aspect of the present invention provides polymers of Formulas 2 and 3. These polymers can be used as intermediate polymers in the preparation of photoresist polymers of Formula 4.
Yet another aspect of the present invention provides a photoresist composition comprising a photoresist polymer described above, a photoacid generator, and an organic solvent.
Preferred photoacid generators have a relatively low light absorbency in the wavelengths of 157 nm and 193 nm. More preferably, the photoacid generator is selected from the group consisting of phthalimidotrifluoromethane sulfonate, dinitrobenzyltosylate, n-decyl disulfone and naphthylimido trifluoromethane sulfonate.
The photoacid generator can further comprise a compound selected from the group consisting of diphenyl iodide hexafluorophosphate, diphenyl iodide hexafluoroarsenate, diphenyl iodide hexafluoroantimonate, diphenyl p-methoxyphenyl triflate, diphenyl p-toluenyl triflate, diphenyl p-isobutylphenyl triflate, triphenylsulfonium hexafluoroarsenate, triphenylsulfonium hexafluoroantimonate, triphenylsulfonium triflate and dibutylnaphthylsulfonium triflate.
Typically, the amount of photoacid generator used is from about 0.1% by weight to about 10% by weight of the photoresist resin employed.
Exemplary organic solvents suitable in PR compositions of the present invention include ethyl 3-ethoxypriopionate, methyl 3-methoxypropionate, cyclohexanone, propylene glycol methyl ether acetate, n-heptanone and ethyl lactate. The amount of solvent used is preferably in the range of from about 400 % to about 1500% by weight of the PR polymer. This ratio has been found to be particularly useful in obtaining a photoresist layer of a desirable thickness when coated on to a suitable substrate such as a silicon wafer in production of a semiconductor element. In particular, it has been found by the present inventors that when the amount of organic solvent is about 1000% by weight of the photoresist polymer, a photoresist composition layer having 0.2 xcexcm of thickness can be obtained.
The present invention also provides a process for forming a photoresist pattern comprising the steps of:
(a) coating a photoresist composition described above on a substrate of semiconductor device to form a photoresist film;
(b) exposing the photoresist film to light using a light source; and
(c) developing the exposed photoresist film.
The process for forming the photoresist pattern can further include a baking step before and/or after the exposure step (b). Preferably, the baking step is performed at temperature in the range of from about 70 to about 200xc2x0 C.
Exemplary light sources which are useful for forming the PR pattern include VUV (157 nm), ArF (193 nm), KrF (248 nm), EUV (13 nm), E-beam, X-ray and ion beam.
The irradiation energy is preferably in the range of from about 1 mJ/cm2 to about 30 mJ/cm2.
In another embodiment, the present invention provides a semiconductor device, which is manufactured using the photoresist composition described above.
Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting.